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Brighter supernovae make dark energy even murkier

By David Shiga

Long-term changes in the average supernova’s brightness may interfere with measurements of dark energy made by future missions like the proposed Supernova Acceleration Probe (SNAP) (Illustration: SNAP)

The average brightness of stellar explosions that astronomers rely on to measure dark energy – the mysterious force causing our universe to expand faster and faster – has actually changed over time, a new study reports. The authors say uncertainties in gauging the brightness might throw off future measurements of dark energy in unpredictable ways.

To measure the expansion of the universe and discern the effects of dark energy, scientists rely on explosions called type Ia supernovae, which are thought to signal the deaths of white dwarf stars.

These explosions can vary in brightness, but their brightness is correlated with how long they last. That means astronomers can theoretically distinguish between bursts that are bright but distant and those that are faint but close, allowing them to be used as ‘standard candles’ whose brightness can be used to estimate astronomical distances.

But even though researchers knew their brightness could vary, they assumed that the overall ratio of different ‘wattages’ remained constant over the history of the universe. Now, a new study led by Andrew Howell of the University of Toronto in Canada suggests that explosions in the early universe were brighter on average than those occurring today, casting doubt on their use as accurate distance gauges.

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High precision

The team pored over data from the Supernova Legacy Survey and the Higher z Supernova Search. They found that brighter supernovae, which last longer than dimmer ones, were more common further back in time than they are today – a finding that may lend support to the notion that there are multiple ways to create the explosions.

The researchers calculate that supernovae were on average 12% brighter 8 billion years ago than they are now.

The effect is not large enough to prompt scientists to question the existence of dark energy. But it suggests that the corrections that have been used in the past to calculate a supernova’s intrinsic brightness from its duration may limit the precision of future supernova surveys.

That could severely hinder cosmologists’ understanding of dark energy, since high-precision observations are needed to find out whether the strength of dark energy has changed over time – a key to determining whether it is an unchanging property of space itself or a varying energy field.

Large samples

Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, US, who led one of the two teams that independently discovered dark energy in 1998 using supernovae, says the shift in the average supernova brightness could affect dark energy measurements.

But only if the corrections that are already in use are not quite right. “We don’t know that there are inaccuracies in the corrections,” he told New Scientist. “The problem is if after the corrections there are still errors.”

Howell says it is unclear whether precise enough corrections can be made. “If we are going to make the next leap to measuring changes in dark energy with time, it requires a correction to better than 2%,” Howell told New Scientist. “We don’t have enough supernovae discovered yet to be able to tell if that kind of precision is achievable.”

Stellar birthrates

Even if this produces errors, there are ways to combat them, Riess says. For example, with enough supernovae, one could do separate calculations using either the brighter supernovae or the dimmer ones. If the results agreed, then it would be safe to conclude that the corrections are working properly and the dark energy calculations are sound.

Alternative methods of measuring dark energy – such as looking at the large-scale distribution of galaxies in space – could also provide an independent test of supernovae studies.

Why the early universe had more of the brighter type Ia supernovae remains a mystery. But one clue comes from observations showing that the brighter ones seem to occur more often where stars are forming at a high rate – and stellar birthrates were higher in the early universe.